A voltage-controlled oscillator (VCO) is an electronic oscillator in which the oscillation frequency is controlled by a voltage input. Voltage controlled oscillators are commonly used in modern communication devices. The variation of the oscillation frequency of the VCO is linear with respect to the input voltage. The change in the output frequency relative to the change in the input control voltage signal (i.e. delta frequency/delta volt) determines the frequency gain of the VCO.
One of the common applications for a VCO is phase locked loop (PLL) circuit. A phase-locked loop is a closed-loop feedback control system that maintains a generated signal in a fixed phase relationship to a reference signal. PLLs are commonly used in demodulation of amplitude modulated signals and angle modulated signals. A PLL circuit generally consists of a phase-frequency detector, a charge pump, a loop filter, a voltage controlled oscillator and an adjustable divider.
The working of a PLL is similar to a feedback system. Here, the phase of the output signal is fed back and the VCO frequency gets adjusted until it is equal to that of the input signal. The phase-frequency detector generates a voltage signal which represents the difference in phase and frequency between two signal inputs. The charge pump uses energy storage elements to convert direct current (DC) voltages into other DC voltages. The loop filter is a low-pass narrow-band filter which is used to remove jitter from the charge pump output. If the VCO needs to operate at a higher frequency, the charge pump drives current into the loop filter. But if the VCO needs to operate at a lower frequency, the charge pump draws current from the loop filter. The loop filter output voltage is given as input to the VCO, according to which the VCO oscillates at a higher or lower frequency. The oscillator output is given as a feedback signal to the adjustable divider which is used to make the PLL's output clock a fractional multiple of the reference frequency at the input of the PLL.
These devices extensively use transistors whose properties vary according to variations in temperature. Transistor operating characteristics get degraded at high temperatures and the frequency gain is relatively low at high temperatures. However at lower temperatures, the VCO frequency gain is relatively high. This large frequency gain variation adversely affects the PLL performance. Transistor properties also vary due to variations in the manufacturing process. These variations in transistor properties lead to performance degradation and unreliable behavior in PLLs.
Prior approaches have been utilized to provide gain compensation for a VCO in PLLs. One such scheme involves making changes in the circuit of the VCO to provide gain compensation. However, this scheme achieves gain compensation only across temperature variations, whereas no compensation is achieved across process variations. Making changes in the circuit of the VCO adversely affects design flexibility and also degrades VCO performance as this VCO design results in more phase noise.